Boris Johnson has accepted Pippa Middleton's challenge to a game of table tennis, although the London Mayor labelled the party planner "just the sister of a duchess".

Pippa - the younger sister of Kate, the Duchess of Cambridge - had said she wanted to play against Johnson to see who had the most competitive family, and he has agreed to go ahead with the game in the hope it can encourage more people to get involved with sport.

After accepting the challenge on Twitter, Johnson said: "I feel very nervous because she seems to be very good. Well, she claims to be very good.

"But I'm very happy to do this because it's a chance to promote what we're doing with sports legacy London.

"We have greatly increased the number of people doing sports. If Pippa's kind offer can be used to promote that then I'm all for it."

However, Johnson appeared to insult his challenger.

He said: "Pippa Middleton. She's a duchess is she? Or isn't she just a sister of a duchess?"

Pippa had initially challenged the mayor to a game in a column for The Spectator magazine.

She wrote: "I'm informed that Boris Johnson, former editor of this magazine, wants to be 'whiff-whaff' world king even more than he wants to be prime minister.

"I'm also told the Johnsons are almost as competitive as the Middletons. So I'd like to lay down a challenge to the mayor... bring it on, Boris."